This invention relates to methods and apparatus in which permanent magnets are employed to mark hidden objects so that the objects may be located, traced, and identified. The invention has particular application to buried fiber optic cables and to other buried non-conductive objects, such as ducts, conduits, or pipes used by utilities. The invention will be described, for purposes of illustration only, as applied to ducts for fiber optic cables.
Many of the fiber optic cables currently being placed in the ground are totally non-conductive. Even the strength member is non-metallic. Conductors that might be used for tracing a buried cable are omitted to prevent lightning from following the conductors and damaging the cable. The absence of conductors creates problems when it becomes necessary to locate a buried cable, because cable tracers require a conductor that carries a tracing signal.
It has previously been proposed to associate permanent magnets with a fiber optic cable, as by providing the magnets on a tracer tape buried separately along with the cable, in order to assist in locating and tracing the cable. However, this approach has several disadvantages, including the possibility that the tracer tape will be installed improperly and thus not detected, the limited distance at which the tape can be detected, and the inability reliably to distinguish the weak magnetic field of the magnets from magnetic fields associated with ferrous pipes, for example, that may be buried in the same area.